In an internal combustion engine, in particular a Diesel engine, it has been noted that knowledge of the value of the pressure within each cylinder makes it possible to better control the progress of the combustion in the engine. This information is then used to regulate the injection of fuel in each of the cylinders. The polluting emissions of the engine may thus be reduced and the consumption optimized.
In an engine of Diesel type, each cylinder comprises a glow plug which makes it possible to heat the inside of the corresponding combustion chamber, in particular on starting the engine. This glow plug is disposed in a threaded bore which passes through the cylinder head of the engine. IT is already known to the person skilled in the art to integrate a pressure sensor into a glow plug. In this way, the bore formed in the cylinder head to receive the glow plug also serves to accommodate a device for measuring the pressure within the combustion chamber. Forming an additional bore in the cylinder head is thus avoided. Forming such a bore would create numerous problems due in particular to the lack of space as well as problems of cost, fluid-tightness, accessibility, etc.
A glow plug generally comprises a tubular body having on its outer surface a threaded portion enabling it to be fixed into a corresponding bore formed in the cylinder head. One portion of that body is then located within the combustion chamber and the other portion outside it. The portion within the combustion chamber bears a finger within which is located a heater electrode. This electrode is supplied by a core which passes through the tubular body of the glow plug. The portion outside the combustion chamber is also termed glow plug head. At that head, the core is connected to a source of electrical energy. It is also known to situate a pressure sensor at that head. This sensor then measures the pressure exerted on the finger of the glow plug. The principle is to measure the force exerted on the finger by the ambient pressure in the combustion chamber by a measurement of the stresses between the body of the glow plug and the finger. FIG. 1 appended hereto represents an embodiment of a glow plug of the prior art provided with a pressure sensor.
In this embodiment of the prior art, described in more detail below, the pressure sensor is disposed between the body of the glow plug on which it bears and a nut fastened to the upper end of the core supplying the pre-heating electrode with electrical energy. In this embodiment, the core passes through the sensor.
Such a glow plug of the prior art makes it possible to measure the pressure exerted within the combustion chamber on the glow plug finger. However, other signals become superposed on the signal corresponding to the pressure measured. To make a correct measurement of the ambient pressure within the combustion chamber of the cylinder considered, the background noise perturbing the pressure measurement should be excluded. This noise arises for example from the deformations of the cylinder head in which the glow plug is mounted.